Semiconductor production apparatus

ABSTRACT

A semiconductor production apparatus for etching a semiconductor wafer arranged in a container and having a film on the surface thereof, by a plasma generated in the container is disclosed. The temporal change of the amount of light is detected for at least two wavelengths obtained from the wafer surface for a predetermined period of the processing time. The etching condition is determined by comparing a predetermined time with the time length between a time point at which the temporal change amount of the light of one of the two wavelengths assumes a maximum value and a time point at which the amount of light of the other wavelength assumes a minimum value.

CROSS-REFERENCE TO RELATED APPLICATION

The present invention is related to (1) U.S. patent application Ser. No.10/230,309 filed Aug. 29, 2002 entitled “SEMICONDUCTOR FABRICATINGAPPARATUS AND METHOD AND APPARATUS FOR DETERMINING STATE OFSEMICONDUCTOR FABRICATING PROCESS” and (2) U.S. patent application Ser.No. 10/377,823 filed Mar. 4, 2003 entitled “SEMICONDUCTOR FABRICATINGAPPARATUS WITH FUNCTION OF DETERMINING ETCHING PROCESSING STATE”, theentire contents of which is incorporated herein by reference for allpurposes.

BACKGROUND OF THE INVENTION

The present invention relates to an apparatus for producingsemiconductor devices by etching, or in particular to a semiconductorproduction apparatus having the function of determining the state of theetching process such as the etched depth.

In producing a semiconductor device, the dry etching process is widelyused in order to remove the layers of various materials such as adielectric material and an insulating material formed on the surface ofa semiconductor wafer or to form a pattern on these layers. In executingthe dry etching process, it is crucial to adjust the etching to securethe desired etching depth and the desired film thickness of the layers.For this reason, the end point of etching and the film thickness arerequired to be detected accurately.

In the process of dry etching a semiconductor wafer using a plasma, thelight emission intensity of the light having a specified wavelengthcontained in the plasma light is known to change with the progress ofthe etching process for a specific layer. To detect the etchingconditions such as the etching end point for the semiconductor wafer orthe layer thickness, a technique is known in which the change in lightemission intensity of a specific wavelength is detected from the plasmaduring the dry etching process, and based on the result of thisdetection, the thickness of a specific layer and the end point ofetching the specific layer are detected. In order to improve theaccuracy of this detection, it is necessary to reduce the detectionerror attributable to the change in the detected waveform due to noises.

In recent years, the ever decreasing size and the resulting everincreasing integration of the semiconductor have reduced the openingratio (the etched area of the semiconductor wafer), and the lightemission intensity of a specific wavelength retrieved from the lightsensor to the light detector is reduced to a very small level. As aresult, the level of the sampling signal from the light detector isreduced, so that it has become more difficult for an end pointdetermining unit to detect the end point of the etching processaccurately based on the sampling signal from the light detector.

Also, when stopping the process by detecting the end point of theetching process, it is actually important that the remaining thicknessof the dielectric layer be equal to a predetermined value. In theconventional method, the whole process is monitored by use of atime-thickness control technique based on the assumption that theetching rate of each layer is constant. The value of the etching rate isdetermined, for example, by processing a sample wafer in advance. Inthis method, the etching process is stopped upon the lapse of a timelength corresponding to a predetermined film thickness (the remainingfilm thickness in the etching process) by the time monitor method.

However, an actual film such as a SiO₂ layer formed by the LPCVD method,for example, is known to have a low thickness reproducibility. Thetolerable error of thickness due to the variation during the LPCVDprocess corresponds to about 10% of the initial thickness of the SiO₂layer. According to the time monitor method, therefore, the actual finalthickness of the SiO₂ layer remaining on the silicon substrate cannot bemeasured accurately. The actual thickness of the remaining layer isfinally measured by a standard spectroscopic interferometer, and in anexcessively etched case, the wafer involved is disposed of as a failure.

Techniques for detecting the end point of the semiconductor waferetching process by measuring the wafer surface using the interferometerare disclosed in JP-A-5-179467 (reference 1), U.S. Pat. No. 5,658,418(reference 2), JP-A-2000-97648 (reference 3) and JP-A-2000-106356(reference 4).

In the technique disclosed in JP-A-5-179467 (reference 1), theinterference light (plasma light) is detected using three color filtersof red, green and blue thereby to detect the etching end point. In theU.S. Pat. No. 5,658,418 (reference 2), on the other hand, the extremevalues of the interference waveform (maximum and minimum points of awaveform as a zero-cross point of the differential waveform) are countedusing the temporal change of interference waveforms of two wavelengthsand differential waveforms thereof. By measuring the time length beforethe count reaches a predetermined value, the etching rate is calculated,and based on the calculated etching rate, the remaining etching timebefore a predetermined film thickness is reached is determined therebyto stop the etching process.

In the technique disclosed in JP-A-2000-97648 (reference 3), on theother hand, a waveform representing the difference (with the wavelengthas a parameter) between the light intensity pattern (with the wavelengthas a parameter) of the interference light before processing and thelight intensity pattern of the interference light after or duringprocessing is determined, and a step (film thickness) is measured bycomparing the particular difference waveform with the differencewaveform stored in a data base. According to JP-A-2000-106356 (reference4) relating to a rotary coating apparatus, the film thickness isdetermined by measuring the temporal change of the interference lightfor multiple wavelengths.

In stopping the process by detecting an etching end point, it isimportant that the remaining film thickness is actually equal or as nearto a predetermined value as possible. In the prior art, the filmthickness is monitored by adjusting the time based on the assumptionthat the etching rate for each layer is constant. The value of thereference etching rate is determined in advance by processing a samplewafer. In this technique, the etching process is stopped upon the lapseof a time length corresponding to a predetermined film thickness.

SUMMARY OF THE INVENTION

In the case where a small number of semiconductor wafers having amultiplicity of different film structures are processed and fabricatedat a time, however, a data base of the multi-wavelength differentialinterference patterns is required to be prepared for each wafer to beprocessed as a product. A test etching process conducted using a waferhaving the same film structure as an actual wafer would increase thewafer cost, and require as many extraneous wafers as the test processes.In the small-volume production, the problem of a high test cost is posedwhich increases the device production cost.

In the prior art described above, a sample wafer for thicknessmeasurement is also required to set the operating conditions of asemiconductor production apparatus by detecting the film thickness ofthe wafer to be processed and to process the product wafer using thethickness detection result. For example, an arbitrary wafer isselectively measured from each lot as a sample wafer. As a result, theextraneous measurement time and wafer are required for a reducedthroughput of the semiconductor production.

An object of this invention is to provide a semiconductor productionapparatus capable of fabricating semiconductor devices at a lower cost.

Another object of the invention is to provide a semiconductor productionapparatus with an improved processing throughput.

The aforementioned objects are achieved by a semiconductor productionapparatus for etching a semiconductor wafer arranged in a container andhaving a film on the surface thereof, using a plasma generated in theparticular container, comprising a detector for detecting the temporalchange of the amount of the light of at least two wavelengths obtainedfrom the wafer surface for a predetermined length of the processingtime, and a determining means for determining the state of the etchingprocess by comparing a predetermined value with the time length betweena time point when the temporal change of the amount of the light of oneof the two wavelengths described above assumes a maximum value and atime point when the light amount of the light of the other wavelengthbecomes minimum.

Further, the determining means determines the thickness of the etchedfilm upon determination that the time length described above becomesequal to or shorter than the predetermined value.

Furthermore, the determining means stops the etching process upondetermination that the time length described above becomes equal to orshorter than the predetermined value.

The invention is also achieved by a semiconductor production apparatusfor etching a semiconductor wafer arranged in a container and having afilm on the surface thereof, using a plasma generated in the particularcontainer, comprising a detector for detecting the interference of thelight from the wafer surface for a predetermined time length of theetching process, a means for comparing a predetermined value with thetime length between a time point when the temporal change of the lightamount of one of at least two wavelengths output from the detectorbecomes maximum and a time point when the light amount of the otherwavelength becomes minimum, and a control unit for adjusting the etchingprocess in response to the output from the comparator means.

Further, the control unit stops the etching process in the case wherethe time length becomes equal to or shorter than the predeterminedvalue.

The invention is also achieved by a semiconductor production apparatusfor etching a semiconductor wafer arranged in a container and having aplurality of films including a first film formed on the surface of thesemiconductor wafer and a second film formed above the first film, byuse of a plasma generated in the container, comprising a light detectorfor detecting the temporal change of the light amount of a plurality ofwavelengths obtained from the wafer surface during a predetermined timelength when the second film is etched, and a means for detecting thethickness of the first film based on a specific waveform obtained fromthe output of the detector.

Further, the detection means detects a unique change of the output ofthe light detector upon detection by the detector of the temporal changeof the amount of the interference light from the wafer surface for aplurality of wavelengths.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal sectional view and a block diagram showing ageneral configuration of a semiconductor production apparatus accordingto a first embodiment of the invention.

FIG. 2 is a schematic diagram showing an outline of the configuration ofa wafer to be processed and the light interference according to thefirst embodiment.

FIG. 3 is a graph showing an example of the data obtained using thelight interference according to the first embodiment.

FIG. 4 is a diagram showing the operation flow of the processing unitshown in FIG. 1, or especially, the operation flow for adjusting theetching process by detecting the etching condition of the member to beprocessed.

FIGS. 5A to 5D are diagrams showing the undercoating oxide filmdependency of the polysilicon film thickness at which the differentialvalue of the interference light data crosses zero and the undercoatingoxide film dependency of the wavelength at which the differential valuefor the particular thickness crosses zero according to the embodimentshown in FIG. 1.

FIG. 6 shows the polysilicon film thickness dependency of the wavelengthat which the interference waveform crosses zero from positive tonegative or from negative to positive with an undercoating oxide filmgroup of 1 nm to 10 nm.

FIG. 7 shows a target film thickness area and an approximation line ofthe zero-crossing wavelength for a the undercoating oxide film thicknessgroup of 270 nm to 340 nm.

FIG. 8 is a diagram showing the operation flow using the result ofpolysilicon interference waveform analysis for the semiconductorproduction apparatus according to the embodiment shown in FIG. 1.

FIG. 9 is a diagram showing the undercoating oxide film dependency ofthe polysilicon film thickness determined by the semiconductorproduction apparatus according to the embodiment shown in FIG. 1.

FIG. 10 is a diagram showing the operation flow of a semiconductorproduction apparatus according to another embodiment of the invention.

FIG. 11 is a graph showing an approximation line of the wavelength ofthe interference waveform.

FIG. 12 is a graph showing an approximation line of the wavelength ofthe interference waveform.

FIG. 13 is a graph showing an interference waveform obtained from asemiconductor production apparatus according to another embodiment ofthe invention.

FIGS. 14A and 14B are graphs showing an interference waveform obtainedfrom a semiconductor production apparatus according to still anotherembodiment of the invention.

FIG. 15 is a flowchart showing the operation flow of the semiconductorproduction apparatus according to the embodiments shown in FIGS. 13 and14.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of this invention are explained below with reference to theaccompanying drawings.

In each of the embodiments described below, those component parts havingthe same or similar functions as or to those of the first embodiment aredesignated by the same reference numeral, respectively, as in the firstembodiment and are not described in detail. In the embodiments describedbelow, an explanation is given about a method of measuring the etchingconditions including the etching amount (etched depth or etched filmthickness) in the process of etching a wafer constituting a member to beprocessed by a semiconductor production apparatus according to thisinvention.

In the description that follows, the term “film thickness” is defined asthe remaining film thickness in the etching process.

Embodiment 1

A first embodiment of the invention is explained below with reference toFIGS. 1 to 3. FIG. 1 includes a longitudinal sectional view and a blockdiagram showing a general configuration of a semiconductor productionapparatus according to a first embodiment of the invention. FIG. 2 is aschematic diagram showing the configuration of a wafer to be processedand an outline of the light interference according to the firstembodiment. FIG. 3 is a graph showing an example of data obtained usingthe light interference according to the first embodiment.

First, with reference to FIG. 1, an explanation is given about a generalconfiguration of a processing unit having a film thickness measuringunit for executing the process of etching a semiconductor wafer sampleaccording to this invention.

A processing unit 1 for executing the etching process includes a vacuumcontainer 2. The gas introduced into the vacuum container 2 is convertedinto a plasma 3 by the electromagnetic wave such as a microwave. Asample 4 constituting a member to be processed such as a semiconductorwafer on a sample table 5 is etched by the plasma 3. According to thisembodiment, the multi-wavelength light radiated from a measurement lightsource (such as a halogen light source) of the film thickness measuringunit 10 is led into the vacuum container 2 by an optical fiber 8 andapplied to the sample 4 at an incidence angle substantially at rightangles thereto. The member to be processed 4 is a substantially circularflat plate of a semiconductor including a thin film layer of polysiliconin the case under consideration. The radiated light is split into thelight reflected and refracted on the upper surface of the polysiliconlayer and the light reflected and refracted at the boundary surfaceformed between the polysilicon layer and an undercoating layer. Bothradiated light combine to form an interference light radiated upward ofthe sample 4. The interference light is led to a spectrometer 11 of thefilm thickness measuring unit 10 through the optical fiber 8, and basedon the detected condition, the film thickness is detected and theprocess executed to determine an end point.

The film thickness measuring unit 10 includes a spectrometer 11, firstdigital filter circuits 12, 22, differentiators 13, 23, second digitalfilter circuits 14, 24, differential waveform data bases (differentialzero-cross time point accumulators) 15, 25, a differential waveformcomparator (differential zero-cross time comparator) 16, a processingcondition determinator (end point determinator) 26 and a result display17.

In the semiconductor production apparatus according to this embodiment,the member to be processed such as a semiconductor wafer is etched witha plasma in such a manner that a wavelength group of which thedifferential value of the interference light for a predetermined filmthickness crosses zero (or assumes a maximum or minimum value) isselected in advance using the result of calculating the lightinterference waveform from the optical property values of the member tobe processed, and the data on a wavelength group λ1 crossing zero frompositive to negative (or assuming a maximum value) and a wavelengthgroup λ2 crossing zero from negative to positive (or assuming a minimumvalue) are stored or recorded in a storage unit or a recording unit ofor communicable with the semiconductor production apparatus. In actuallyprocessing the member to be processed 4, the intensity of theinterference light of the wavelength groups λ1 and λ2 is measured. Atime point at which the differential value of the interference lightintensity measurement of each wavelength group crosses zero (assumes anextreme value) is detected. This zero-crossing time point is comparedwith a predetermined value thereby to determine the film thickness ofthe member to be processed.

The light emission intensity of a given wavelength included in thewavelength group λ1 retrieved by the light detector 11 is converted intoa voltage signal from a current detection signal corresponding to thelight emission intensity thereof. The signal of a plurality of specificwavelengths output as a sampling signal from the spectrometer 11 isaccommodated as a time series data yij in a storage unit such as a RAM.Next, the time series data yij is smoothed by the first digital filtercircuit 12, and stored as a smoothed time series data Yij in a storageunit such as a RAM. Based on the smoothed time series data Yij, the timeseries data dij of the differential coefficient value (firstdifferential value or second differential value) is calculated by thedifferentiator 13, and stored in a storage unit such as a RAM. Thedifferential coefficient time series data dij is smoothed by the seconddigital filter circuit 14, and stored as the smoothed differentialcoefficient time series data Dij in a storage unit such as a RAM. Fromthis smoothed differential coefficient time series data Dij, a realpattern of the differential values of the interference light intensityof each wavelength is determined.

The light emission intensity of a given one wavelength contained in thewavelength group λ2 retrieved by the light detector 21, on the otherhand, is converted into a voltage signal from a current detection signalcorresponding to the light emission intensity thereof. A signal of aplurality of specific wavelengths output as a sampling signal from thespectrometer 11 is stored as a time series data y′ij in a storage unitsuch as a RAM. Next, this time series data y′ij is smoothed by the firstdigital filter circuit 22, and stored as a smoothed time series dataY′ij in a storage unit such as a RAM. Based on the smoothed time seriesdata Y′ij, the time series data d′ij of the differential coefficientvalue (first or second differential value) is calculated by thedifferentiator 23 and stored in a storage unit such as a RAM. Thedifferential coefficient time series data d′ij is smoothed by the seconddigital filter circuit 24, and stored as a smoothed differentialcoefficient time series data D′ij in a storage unit such as a RAM. Fromthis smoothed differential coefficient time series data D′ij, a realpattern of the differential value of the interference light intensity ofeach wavelength is determined.

The differential data storage units 15, 25 have stored therein theinterference light intensity data of each wavelength with the change infilm thickness or the value and the time point of each waveform of thesedifferential data. Especially, the maximum and minimum values providinga crest and a bottom of the interference waveform (i.e., the zero-crosspoint of the first differential data waveform) and the time points Tm,T′n thereof are stored. The time length between the time points Tm, T′nis preset in the relational operator 16 or compared with a predeterminedvalue stored or recorded in a storage or a recording unit. In the casewhere the time length between the time points Tm and T′n is shorter thanthe predetermined value, it is determined that the remaining filmthickness has reached or substantially reached a predetermined size, andthe remaining film thickness of the member to be processed isdetermined. The result is displayed on the display 17.

The present inventors have discovered that when etching a film formed ona semiconductor wafer, a plurality of wavelength groups with one of theinterference waveform data thereof assuming a maximum value and theother of the interference waveform data thereof assuming a minimum valuecan be selected in the neighborhood of the time point associated with aspecified film thickness, and that the time length between the timepoints at which the waveform of the wavelength associated with thesegroups assumes the maximum and minimum values is decreased with thedecrease in film thickness. As a result, the present inventors have comeknow that it can be determined whether a specific film thickness hasbeen reached or not by presetting the interval between a time point whena maximum value is assumed and a time point when a minimum value isassumed as a reference for the specific film thickness.

The differential data storage unit 15 has stored therein the change ofthe differential waveform data of the interference light from the sample4 providing a member to be processed on the one hand and the time pointswhen the interference waveform data assume extreme values (maximum andminimum values, i.e. the zero-cross points of the differential datawaveform) on the other hand. Specifically, the data on the time pointsof the crest and the bottom of the interference light intensity due tothe change in film thickness are stored. Also, from the differentialdata storage unit 15, the data on the differential values of theinterference light during a predetermined etching processing time for apredetermined range of wavelength are transmitted to and displayed onthe display 17. Further, according to this embodiment, the data on thetime points when the differential values of a plurality of interferencelight wavelengths pass zero (as a maximum value of interference waveformwhen the first differential value crosses zero from positive to negativesides, and as a minimum value of interference waveform when the firstdifferential value crosses zero from negative to positive sides) arealso adapted to be displayed on the display 17.

The film thickness of the member to be processed is also determined bycomparing the time point Tm with the time point T′n by the differentialwaveform comparator 16. The result of comparison is displayed on theresult display 17.

This embodiment represents a case involving only one light detector 11.In the case where it is desired to measure and control the internalsurface of the member to be processed over a wider area, however, aplurality of light detectors 11 may be used.

FIG. 2 is a longitudinal sectional view of the sample 4 providing amember to be processed such as a semiconductor wafer used for gateetching as an example of the process executed by the processing unitaccording to this embodiment. In FIG. 2, the member to be processed(polysilicon) 40 constituting a film to be processed is formed on anoxide film 42 providing an undercoating material on the sample (wafer)4. Further, a mask 41 is stacked on the member to be processed 40. Inthe case where a gate film is etched on the sample 4, for example, theundercoating material of the member to be processed 40 is an insulatingfilm of SiO₂, and a gate layer of polysilicon is formed on thepolycrystal undercoating material between the source and the drain.Also, a device isolating groove 49 for guaranteeing the independentoperation of the gate electrode 48 of each device is formed by an oxidefilm. Further, according to this embodiment, the gate electrode 48 isformed on the side of the device isolation groove (shallow trenchisolation, STI) 49 of the oxide film 42 under the mask 41.

The surface of the sample 4 having this configuration is irradiated withthe light having a plurality of wavelengths emitted from thespectrometer 11 or the plasma 3, which light enters the sample 4including a stack structure of the member to be processed 40 and theoxide film 42 providing an undercoating material substantially at rightangles thereto. The radiated light which is led to the gate electrode 48having a thin oxide film (undercoating material) 42 contains a radiatedlight component reflected on the upper surface of the member to beprocessed 40 and a radiated light component reflected on the boundarysurface formed between the member to be processed 40 and theundercoating material 42. An interference light 95A is formed by theradiated light reflected at these points and radiated upward of thesample 4. In similar fashion, the radiated light which is led to thedevice isolation portion 49 having a thick undercoating material 42contains a radiated light component reflected on the upper surface ofthe member to be processed 40 and a radiated light component reflectedon the boundary surface formed between the member to be processed 40 andthe undercoating material 42. An interference light 95B is formed by theradiated light reflected at these points and radiated upward of thesample 4. These interference light decrease in interference intensitywith the decrease in the thickness of the undercoating oxide film, andtherefore the intensity of the interference light 95B is greater thanthat of the interference light 95A.

The interference light reflected is led to the spectrometer 11 therebyto generate a signal changing in intensity with the thickness of thelayer of the member to be processed 40 during the etching process. Amongthe interference light detected through the spectrometer 11, theinterference light 95B from the thick film portion of the undercoatingmaterial 42 is more controlling than the interference light 95A.According to this embodiment, the etching factors such as the filmthickness and the etched groove depth are detected by the interferencelight with higher accuracy for those on the device isolation groove 42(the film thickness 46, etc.).

The display 17 may use a liquid crystal or a CRT or may be replaced orcombined with an announcing unit for informing by light or sound that apredetermined film thickness or the end point has been reached.According to this embodiment, the apparatus comprises a display fordisplaying the measurement data as a graph and the display 17 having aunit using light or sound for announcement.

Further, the apparatus according to this embodiment has the function ofdisplaying the specific information desired by the user who has visuallyrecognized the measurement data indicated on the display 17 or renderingthe user to designate the information required to detect or calculatespecific information. The function such as a pointer for designating aspecific or an arbitrary point on the time-wavelength coordinateindicated on the display 17 or the data thereof, the function ofcalculating or detecting the data values at the designated points andspecific amounts indicating the etching conditions including the timelength between specific time points, the wavelength, the etching rateand the film thickness from these data values, and the function ofdisplaying these amounts at a predetermined position in a way easilyrecognizable by the user, are some examples.

The unit for calculating the above-mentioned amounts may be an operatorincluded in the apparatus or another operator arranged at a remote placewith which the apparatus can transmit and receive the measured ordetected data through a communication unit.

In FIG. 1, a functional configuration of the device for measuring theetching amount is shown. The actual configuration of the measuring unit10 except for the display 17 and the spectrometer 11 may include a CPU,a ROM for holding various data including the program for measuring theetching depth and the film thickness and a differential waveform patterndata base of the interference light, a RAM for holding the measurementdata, a storage device including an external storage, a datainput/output device, and a communication control unit. This is also thecase with other embodiments described below.

FIG. 3 is a graph showing the temporal change of the waveform data ofthe interference light detected by the semiconductor productionapparatus according to this embodiment for a plurality of wavelengths.As shown in FIG. 3, it is understood that in a combination of severalwavelengths, a wavelength can be selected in which the data assumes aminimum value at about the time point when one of the wavelengthsassumes an extreme value (maximum value). The present inventors havecome to know that the time difference between the time points when thetwo wavelengths assume a maximum value and a minimum value,respectively, is reduced with the progress of the process (with thereduction in the remaining film thickness), and that this timedifference and the film thickness are correlated and by determining thetime difference, it is possible to determine the remaining filmthickness (etching (groove) depth) and an end point of the process. Thisidea has been incorporated in the invention.

FIG. 4 is a flowchart showing the operation flow of the processing unitshown in FIG. 1, or especially, the operation flow for adjusting theetching process by detecting the etching conditions of the member to beprocessed.

According to this embodiment, the semiconductor production apparatusacquires in step 800 the conditions for etching the polysilicon filmproviding the member to be processed 40. In this step, the informationmay be received from the data base of the processing conditions storedor recorded in a storage unit or a recording unit in advance, or theinformation may be received which is input by an input device such as akeyboard or a mouse of the display 17 by the user. As anotheralternative, the data indicating the film configuration recorded in thewafer 4 or the cassette accommodating the semiconductor wafer 4 inadvance may be acquired and detected by an operating unit or the likenot shown.

Next, in step 401, by use of the data stored in the differential datastorage unit or by comparing each waveform stored or recorded in anotherstorage or recording unit with the differential data of each intensity,the wavelength groups λ1 and λ2 for determining the etching conditionare detected. Further, a time difference ΔT providing a reference of thetime difference between the time points at which the first differentialcrosses zero is set.

In steps 402, 403 and 404, the process of the wafer 4 is actuallystarted and the waveform data of the interference light obtained duringthe process is detected. At the same time, the interference waveform ofthe determining wavelength groups λ1, λ2 set in step 401 aredifferentiated thereby to calculate the time points T1, T2 at which thedifferential data of each wavelength group cross zero.

In step 405, the time difference (T1−T2 or T2−T1) between T1 and T2calculated in step 404 is compared with the time difference ΔT providinga reference set in step 401, by use of the comparator 16. Upon judgmentthat the relation T1−ΔT≦T2≦T1+ΔT fails to be met, i.e. that the timedifference ΔT is smaller than the time difference between T1 and T2, itis determined that the desired film thickness has not reached, and theprocess returns to step 403 thereby to continue processing the member tobe processed 40. In the case where it is determined that the relationT1−ΔT≦T2≦T1+ΔT is met, i.e. that the time difference ΔT is larger thanor equal to the time difference between T1 and T2, on the other hand, itis determined that the film has reached or decreased below the desiredthickness. Then, the process proceeds to step 406 to end the etchingprocess or end the sampling process.

According to this embodiment, the etching is stopped in this step, andso is the sampling of the interference light of the wavelength groupsλ2, λ2 through the spectrometer 11.

The present inventors, as the result of studying the interference of themember to be processed (polysilicon film) 40 taking the effect of theundercoating film (oxide film) 42 into consideration, have found thatthe interference waveform appearing at the time of thickness changeduring the etching process of the polysilicon 40 is affected by thethickness of the undercoating film (oxide film) 4.

FIGS. 5A to 5D are diagrams the dependency of the polysilicon filmthickness on the undercoating oxide film during the etching process inthe film thickness measuring unit shown in FIG. 1, whereby thedifferential value crosses zero from positive to negative side for thedata on the interference light having a measured wavelength of 400 nmand a measured wavelength of 380 nm, and the dependency of thewavelength on the undercoating oxide film whereby the differential valuecrosses from positive to negative for the same film thickness.

FIG. 5A shows the thickness of the polysilicon film 40 about 60 nm thickwith the first differential thereof crossing zero from negative topositive (reaches a minimum value), as determined against the thicknessof the undercoating oxide film for the light having the wavelength of400 nm. As shown in FIG. 5A, the polysilicon film thickness undergoes achange as determined for the undercoating oxide film thickness with aperiodicity of about 130 nm. This is by reason of the fact that theinterference of the polysilicon film is connected so that theinterference of the undercoating oxide film continues. Specifically, theperiodicity is sin(4πnd/λ), where n is the refractive index of theundercoating oxide film, d the thickness of the undercoating oxide film,and λ the wavelength.

FIG. 5B shows a wavelength group of the interference light which crosseszero from positive to negative (reaches a maximum value) for thepolysilicon film thickness at which the light having a wavelength of 400nm changes by crossing zero from negative to positive. As shown in FIG.5B, this wavelength group exists over the range of about 430 nm to about500 nm. By measuring the light having the wavelength of 400 nm and thewavelength group of about 430 nm to 500 nm, therefore, the thickness ofthe undercoating oxide film can be roughly determined.

In the case where the extreme values for the wavelength 400 nm and thewavelength 440 nm are coincident with each other, for example, theundercoating oxide film is determined as about several nm to 130 nm,about 170 nm to 260 nm or about 330 nm to 380 nm.

In the case where the zero-cross points of the light having a wavelengthof 400 nm, the light having a wavelength of 440 nm and the light havinga wavelength of 480 nm are coincident with each other, on the otherhand, the undercoating oxide film is determined as about 140 nm to 170nm or about 300 nm to 330 nm. Further, when considering the waferproduct specification, the undercoating oxide film thickness is limitedmore. By use of the undercoating film thickness (about 300 nm to 330 nm)determined in this way, the band of the wavelength group λ2 crossingzero from positive to negative is narrowed from about 410 nm to 420 nm(from about 410 nm to 450 nm) when the measured wavelength 380 nm shownin FIG. 5C crosses zero from negative to positive (FIG. 5D). At the sametime, the polysilicon film thickness thus far determined as about 48 nmto 56 nm is changed to about 52 nm to 55 nm for an improved accuracy.

In the case where it is desired to control the member to be processed bymeasuring the surface thereof over a wide area, a plurality ofspectrometers may be used.

Also, the interference light may be measured by a measuring unit such asa spectrometer 11 using the light from the plasma 3 generated in thevacuum container 2 without using the light source for supplying thelight into the vacuum container 2 as in the embodiment described above.In such a case, the plasma light reflected from the surface of thesample 4 is supplied to the spectrometer 11. In order to measure thechange in the plasma light, a measurement port or an opticaltransmission unit is arranged on the side wall of the vacuum container 2in such a manner as to be capable of receiving the inner light, wherebythe detected signal is used as a reference light. This reference lightis not passed through the light path directly incident from the surfaceof the sample 4, and the change in the light from the plasma 3 isrequired to be detected. According to this embodiment, the light fromthe plasma 3 is received by a photodetector arranged on the side wall ofthe vacuum container 2.

Analysis of the interference of the member to be processed (polysiliconfilm) 40 taking the effect of the undercoating film (oxide film) 42 intoconsideration shows that the interference waveform appearing when thefilm thickness changes at the time of etching the polysilicon 40 can bedivided into groups by the thickness of the undercoating film (oxidefilm) 4, and by selectively setting the groups, the thickness of thepolysilicon film during the etching process can be easily measured.

Next, an embodiment using this simple method is explained. FIG. 6 showsthe polysilicon film thickness dependency of the wavelength for whichthe interference waveform groups for the undercoating oxide filmthickness of 1 nm to 10 nm cross zero from positive to negative or fromnegative positive side. In FIG. 6, the solid lines indicate the firstdifferential of the interference waveform crossing zero from negative topositive side, and the dashed line indicates the first differential ofthe interference waveform crossing zero from positive to negative side.The marks □ indicate the maximum, minimum and average polysilicon filmthickness crossing zero from negative to positive side. As shown in FIG.6, the wavelength for which the first differential crosses zero changessubstantially linearly with the polysilicon film thickness. By usingthese approximation lines, therefore, the wavelength for which the firstdifferential crosses zero can be determined with a target filmthickness. With regard to the target film thickness of 50 nm, forexample, the interference waveform having a wavelength of about 425 nmcrosses zero from negative to positive side, while the interferencewaveform having a wavelength of about 575 nm crosses zero from positiveto negative side.

The rectangular frames (i) to (v) shown in FIG. 6 indicate areas forselecting an approximation line to determine the zero-cross wavelengthfor film thickness determination against a target film thickness.Specifically, a wavelength crossing zero from negative to positive sidefor the target film thickness area (120 to 88 nm) is determined asy=0.4026x−76.488, and as y=0.3141x−62.428 for a wavelength crossing zerofrom positive to negative side, where y is a target film thickness and xa zero-crossing wavelength. Also, the wavelength crossing zero fromnegative to positive side is determined as y=0.2269x−47.479 and thewavelength crossing zero from positive to negative side asy=0.3141x−62.428 for the target film thickness area (88 to 65 nm), thewavelength crossing zero from negative to positive side is determined asy=0.2269x−47.479 and the wavelength crossing zero from positive tonegative side as y=0.1418x−33.871 for the target film thickness area (65to 38 nm), the wavelength crossing zero from positive to negative sideis determined as y=0.1418x−33.871 for the target film thickness area (38to 20 nm), and the wavelength crossing zero from negative to positiveside is determined as y=0.0495x−15.548 for the target film thicknessarea (20 to 10 nm).

By dividing into groups by the undercoating oxide film thickness andselectively setting the approximation line of the zero-crossingwavelength for each target film thickness in this way, an algorithm canbe constructed for more accurate film thickness determination.

FIG. 7 shows approximation lines of the zero-crossing wavelength andtarget film thickness areas for the group having an undercoating oxidefilm thickness of 270 nm to 340 nm. The rectangular frames (i) to (viii)shown in FIG. 7 also indicate areas for selecting an approximation lineto determine the zero-cross wavelength for film thickness determinationagainst a target film thickness. In this group, a theoreticalinterference analysis of the member to be processed (polysilicon film)40 taking the effect of the undercoating film (oxide film) 42 intoconsideration shows that the interference wavelength of about 450 nm to500 nm is distorted by the periodicity (about 130 nm) of interferencedue to the undercoating oxide film. It has thus been found that insetting a film thickness determining wavelength, the approximation lineis required to avoid the wavelength range of about 450 nm to 500 nm toavoid a determination error due to the distortion.

FIG. 8 shows the operation flow of a processing unit using the result ofthe aforementioned interference waveform analysis of polysilicon, orespecially, the operation flow for adjusting the etching process bydetecting the etching condition of the member to be processed.

In FIG. 8, the semiconductor production apparatus according to thisembodiment acquires in step 800 the conditions (the etching dischargeconditions, the target remaining film thickness, and the undercoatingoxide film thickness) for etching the polysilicon film providing amember to be processed 40. Next, in step 801, the wavelengths of the twowavelength groups λ1, λ2 used for determining the etching conditions aredetermined by an approximation line capable of calculating thewavelength of which the first differential of the target film thicknessstored or recorded as a data table of the data base in the storage orrecording device in advance crosses zero from negative to positive sideand an approximation line for calculating the wavelength of which thefirst differential crosses zero from positive to negative side. Further,a reference time difference dT is set for these wavelengths, thezero-cross direction and the time difference between time points atwhich the extreme values in opposite signs are assumed.

In steps 802, 803 and 804, the actual processing of the wafer 4 isstarted, and the interference light waveform data (such as the actualwaveform, the time series data of the waveform after differentiation,etc.) obtained during the same process are detected. At the same time,the interference waveform of the determining wavelength groups λ2, λ2set in step 801 are differentiated. In this way, time points t1, t2 arecalculated at which the differential data of each wavelength crosseszero in opposite signs for each wavelength (i.e. the time points atwhich the one of the first differentials crosses zero from negative topositive side, and the other first differential crosses zero frompositive to negative side).

In step 805, the time difference between T1 and T2 (T1−T2 or T2−T1)calculated in step 804 is compared with the reference time difference dTset in step 801 by a comparator 16. In the case where it is determinedthat the relation |T1−T2|≦dT fails to be met, i.e. that the timedifference dT is smaller than the time difference between T1 and T2, itis determined that the desired film thickness is not reached. Then, theprocess returns to step 803 and the processing of the member to beprocessed 40 is continued. In the case where it is determined that therelation |T1−T2|≦dT is met, i.e. that the time difference dT is largeror equal to the time difference between T1 and T2, on the other hand, itis determined that the film thickness has reached or decreased below thedesired one, so that the process proceeds to step 806 at which theetching end and the sampling end are set.

According to this embodiment, the etching is stopped at this point, andso is the sampling of the interference light of the wavelength groupsλ1, λ2 through the spectrometer 11.

FIG. 9 shows the undercoating oxide film dependency of the polysiliconfilm thickness determined according to an embodiment of this invention.It is seen from FIG. 9 that the polysilicon film thickness (in the rangeof 90 nm to 10 nm) for the undercoating oxide film area of 1 nm to 10 nmcan be determined with an accuracy of about ±2 nm.

Another embodiment of the invention is explained with reference to FIG.10. According to this embodiment, the fact that a film thicker than atarget film thickness has been etched, i.e. a film thickness greaterthan the target film thickness has been passed is detected to improvethe reliability of film thickness determination. For the presentpurpose, the film thickness greater than the target film thickness iscalled a passed film thickness. By detecting the passed film thicknessand starting the determination of the target film thickness, adetermination error which might occur for a small target film thicknessis prevented.

According to this embodiment, the semiconductor production apparatusacquires the conditions (the etching discharge conditions, the targetremaining film thickness and the undercoating oxide film thickness,etc.) for the process of etching a polysilicon film providing the memberto be processed 40. Next, in step 1001, a determining wavelength havingtwo wavelength groups is obtained from the wavelength at which the firstdifferential of the target film thickness stored or recorded in astorage unit or a recording unit crosses zero from negative to positiveside or the information on the approximation line for determining thesame wavelength, and the wavelength at which the first differentialcrosses zero from positive to negative side or the information on theapproximation line for determining the same wavelength. Then, areference time difference dT is set for the wavelengths, the zero-crossdirection thereof and the time difference between the time pointsassuming the opposite extreme values.

Further, with regard to the passed film thickness set in accordance withthe target film thickness, a determining wavelength having twowavelength groups is obtained from the wavelength at which the firstdifferential of the passed film thickness stored or recorded in astorage unit or a recording unit crosses zero from negative to positiveside or the information on the approximation line for determining thesame wavelength, and the wavelength at which the first differentialcrosses zero from positive to negative side or the information on theapproximation line for determining the same wavelength, thereby settingthe particular wavelength and the zero-cross direction thereof. For thetarget film thickness of 10 nm of the undercoating oxide film group of 1nm to 10 nm, for example, the corresponding passed film thickness forthe target film thickness are 100 nm, 50 nm and 40 nm. The determiningwavelength for the passed film thickness 100 nm is set at 430 nm fordetection of a zero cross point from positive to negative side and at510 nm for detection of a zero cross point from negative to positiveside, the determining wavelength of the passed film thickness 50 nm isset at 425 nm for detection of a zero cross point from positive tonegative side and at 580 nm for detection of a zero cross point fromnegative to positive side, the determining wavelength of the passed filmthickness 40 nm is set at 380 nm for detection of a zero cross pointfrom positive to negative side and at 500 nm for detection of a zerocross point from negative to positive side, and the determiningwavelength of the target film thickness is set at 500 nm for detectionof a zero cross point from positive to negative side. By determining thetarget film thickness after detection of the passed film thickness inthis way, the determining wavelength can detect the zero cross pointfrom positive to negative side at 500 nm without error.

In steps 1002, 1003 and 1004, the actual processing of the wafer 4 isstarted and the interference light waveform data obtained during thisprocess is detected. At the same time, the interference waveform of thetwo wavelength groups for the passed film thickness set in step 1001 isdifferentiated thereby to calculate the time points t1 and t2 at whichthe differential data for the respective wavelength groups assumesextreme values.

In step 1005, as described above, the time difference (t1−t2 or t2−t1)between t1 and t2 calculated in step 1004 is compared with the referencetime difference dT set in step 1101, by the comparator 16. In the casewhere it is determined that the relation |t1−t2|≦dT fails to be met,i.e. that the time difference dT is smaller than the time differencebetween t1 and t2, it is determined that the desired passed filmthickness is not reached. Then, the process returns to step 1003 tocontinue the processing of the member to be processed 40. In the casewhere it is determined that the relation |t1−t2|≦dT is met, i.e. thatthe time difference dT is larger than or equal to the time differencebetween t1 and t2, it is determined that the film thickness has reachedor decreased below the desired one. Further, the process proceeds tostep 1006, where it is determined whether all the passed film thicknessthat have been set are passed and a target film thickness has beenapproached or not. In the case where a passed film thickness to bepassed before the target film thickness is set, the process returns tostep 1003 to determine whether the passed film thickness has been passedor not.

In the case where it is determined that all the passed film thicknesshave been passed, the process proceeds to step 1007 where in order todetect whether the target film thickness has been reached or not, likein steps 1000 to 1005, the interference light waveform data obtained byprocessing the wafer 4 is detected, and the interference waveform of thetwo wavelength groups for determining the target film thickness set instep 1001 is differentiated. In this way, the time points T1 and T2 arecalculated at which the differential data assumes an extreme value ineach wavelength group.

In step 1008, the time difference between T1 and T2 (T1−T2 or T2−T1)calculated in step 1107 is compared by the comparator 16 with thereference time difference dT set in step 1001. In the case where it isdetermined that the relation |T1−T2|≦dT fails to hold, i.e. that thetime difference dT is smaller than the time difference between T1 andT2, it is determined that the film thickness has not reached the desiredamount. Then, the process returns to step 1007 to continue theprocessing of the member to be processed 40. Once it is determined thatthe relation |T1−T2|≦dT holds, i.e. that the time difference dT islarger than or equal to the time difference between T1 and T2, on theother hand, it is determined that the film thickness has reached ordecreased below the desired amount. Then, the process proceeds to step1009 to set the end of etching and sampling. According to thisembodiment, the etching is stopped at this point, and so is the samplingof the interference light through the spectrometer 11.

To determine a target film thickness, a plurality of passed filmthickness may be set and the target film thickness is predicted fromthese passed film thickness and the passed time thereof thereby to endthe etching process. For example, the passed film thickness is set ateach 10 nm in the range of 100 nm to 30 nm and the passed time ismeasured for each passed film thickness. A regression line isdetermined, for example, using the passed film thickness and the passedtime, and from this regression line, the time point at which the targetfilm thickness is reached is calculated and used for determination.

In the measurement of the interference waveform, the light amount of alight source for obtaining the interference light or the plasma lightamount may undergo an abrupt change. In such a case, the interferencewavelength other than the two wavelength groups for film thicknessdetermination are measured at the same time, and it is determinedwhether the first differential of the interference waveform assumes apositive (or negative) value or not. This detection can prevent anerroneous determination of the interference wave in the case where asimilar change like that of light described above occurs in all thewavelength ranges.

The wavelength of the interference waveform for preventing the erroneousdetermination of the interference waveform can be determined from theinflection point of the interference waveform, i.e. the point at whichthe second differential crosses zero. FIGS. 11 and 12 show theapproximation lines of the interference wavelength for preventing theerroneous determination of the interference waveform for theundercoating oxide film group of 1 nm to 10 nm and the undercoatingoxide film group of 270 nm to 340 nm.

Other embodiments of the invention are explained below with reference toFIGS. 13, 14A, 14B and 15.

FIGS. 13, 14A and 14B are diagrams showing interference waveformsdetected by the semiconductor production apparatus according to otherembodiments of the invention. In FIG. 13, the left graph shows theintensity change of the interference light with the ordinaterepresenting the wavelength and the abscissa representing the filmthickness (processing time), and the right graph shows the intensitychange of the wavelength range of less than 300 nm to 700 nm or more ata specified time point indicated by dotted line in the left graph.

As shown in FIG. 13, the interference light undergoes a sharp change atabout a specific wavelength with the differential data of the intensitygreatly changing vertically. FIGS. 14A and 14B, like FIG. 13, shows theintensity change of the interference light with the ordinaterepresenting the wavelength and the abscissa the film thickness(processing time). FIG. 14A shows the case in which the thickness of theoxide film 42 formed under the polysilicon providing the member to beprocessed 40 is 290 nm, and FIG. 14B shows the case in which theundercoating oxide film is 330 nm. As seen from these diagrams, thepresent inventors have come to know that the value of the wavelengthwith the intensity of the differential data of the interference lightgreatly changing vertically undergoes a change depending on thethickness of the undercoating oxide film 42, and the wavelength and thethickness of the undercoating oxide film 42 are correlated with eachother, so that the thickness of the underlying oxide film can bedetected using the interference light data obtained at the time ofetching the member to be processed located above the oxide film. Thisembodiment of the invention is conceived based on this knowledge.

FIG. 15 is a flowchart showing the operation flow of a semiconductorproduction apparatus according to an embodiment in which theinterference waveforms shown in FIGS. 13, 14A and 14B are obtained.According to this embodiment, at the time of etching a polysilicon gate,the interference light intensity of a wavelength range of about 300 nmto 700 nm is detected thereby to obtain the differential data thereof.The thickness of the undercoating oxide film is calculated from theinverted waveform crossing zero (assuming an extreme value) when thethickness of the polysilicon film 40 to be processed is reduced, byutilizing not the fact that the property of the particular wavelengthshifts sequentially toward the short wavelength side but the fact thatthe shorter wavelength inverts before the longer one due to theundercoating oxide film thickness.

As a result, the thickness of the underlying film (undercoating oxidefilm) that has thus far been determined by selectively measuring thewafer providing an arbitrary sample can be detected using theinterference light data when the upper film is processed. Therefore, thethroughput that has been reduced for measuring the sample can besuppressed while at the same time preventing the production cost fromincreasing.

Thus, the accuracy can be improved for determining a predetermined filmthickness of the polysilicon providing a member to be processed, usingthe thickness of the undercoating oxide film calculated as describedabove.

FIG. 15 shows a process in which the etching conditions (the etchingdischarge conditions, and the remaining film thickness conditions) forthe member to be processed (polysilicon) are input (step 1500), and thetwo determining wavelength groups λ1, λ2 with the zero-cross directionand the determining time width ΔT are set from the data stored in thedata storage unit based on the remaining film thickness conditions (step1501).

Next, the wafer 4 begins to be etched and sampled (step 1502), thedifferential coefficient time series data Di,j are calculated based onthe multi-wavelength output signal yi,j from the spectrometer (step1503), and the wavelength λi is calculated at which the zero-cross timeTi and Tm of the differential coefficient time series data Di,j and Dm,jare inverted (step 1504), where i indicates the wavelength measured onthe long wavelength side, and m the wavelength measured on the shortwavelength side.

In the wavelength range not affected by the undercoating oxide film, therelation Ti<Tm holds between time Ti and time Tm, while the relationbetween time Ti and time Tm is Ti>Tm in the wavelength range affected bythe undercoating oxide film. The thickness of the undercoating oxidefilm is calculated from the wavelength λi by the distorted wavelengthvalue and the undercoating oxide film thickness held in the data base(step 1505). Using the calculated thickness of the undercoating oxidefilm, the two determining wavelength groups λ1, λ2 are set again fromthe differential zero-cross table (step 1506).

The interference waveform of the two determining wavelength groups λ1,λ2 set again are differentiated (step 1507).

Next, the reference time difference ΔT set in step 901 is compared withthe time difference between the zero-cross time points T1 and T2, i.e.it is determined whether the relation T1−ΔT≦T2≦T1+ΔT is held or not(step 1508) thereby to set the end of etching and sampling (step 1509).In the case where the result of comparison between the time differenceΔT and the time difference between the zero-cross time points T1 and T2(i.e. determination as to whether the relation T1−ΔT≦T2≦T1+ΔT is held ornot in step 1508) is NO, the process is repeated from step 1503.

Once the thickness of the undercoating oxide film can be calculated,however, the process of steps 1503 to 1505 is not necessarily executed.

In the case where the determination in step 1508 is YES, on the otherhand, the process proceeds to step 1509 for setting the end of etchingand sampling.

As described above, unlike in the prior art wherein the processingthroughput is reduced with an increased cost by the measurement using asample wafer, the present invention is such that the thickness of theundercoating film can be detected from the data obtained at the time ofprocessing the upper film as a product. Thus, the overall processingthroughput including the apparatus is improved and the cost increase issuppressed.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. A semiconductor production apparatus for etching a semiconductorwafer arranged in a container and having a film on the surface thereof,by use of a plasma generated in said container, comprising: a detectorfor detecting the temporal change of the amount of light for at leasttwo wavelengths obtained from the surface of said wafer for apredetermined period of said processing time; and a determining meansfor determining the etching condition by comparing a predetermined timewith the time length between a time point at which the temporal changeamount assumes a maximum value for the light of one of said twowavelengths and a time point at which the amount of light for the otherwavelength assumes a minimum value.
 2. A semiconductor productionapparatus according to claim 1, wherein said determining meansdetermines the thickness of said etched film in the case where said timelength is not longer than said predetermined value.
 3. A semiconductorproduction apparatus according to claim 1, wherein said determiningmeans stops said etching process upon determination that said timelength is not longer than said predetermined value.
 4. A semiconductorproduction apparatus for etching a semiconductor wafer arranged in acontainer and having a film on the surface thereof, by use of a plasmagenerated in said container, comprising: a detector for detecting theinterference of the light from said wafer surface during a predeterminedperiod of said etching process; a comparator means for comparing apredetermined value with the time length between a time point at whichthe temporal change of the amount of light having one of at least saidtwo wavelengths output from said detector assumes a maximum value and atime point at which the amount of light having the other wavelengthassumes a minimum value; and a control unit for adjusting said etchingprocess upon receipt of the output of said comparator means.
 5. Asemiconductor production apparatus according to claim 4, wherein saidcontrol unit stops said etching process upon determination that saidtime length is not longer than said predetermined value.
 6. Asemiconductor production apparatus for etching a semiconductor waferarranged in a container and including a plurality of film layers havinga first film formed on the surface of said semiconductor wafer and asecond film formed above said first film, by use of a plasma generatedin said container, comprising: a light detector for detecting thetemporal change of the amount of light having a plurality of wavelengthsobtained from said wafer surface during a predetermined time when saidsecond film is etched; and a detection means for detecting the thicknessof said first film based on a specific waveform obtained from the outputof said detector.
 7. A semiconductor production apparatus according toclaim 6, wherein upon detection by said detector of a temporal change ofthe amount of the interference light from said wafer surface for aplurality of wavelengths, said detection means detects a unique changeof the output of said light detector.